Fundus imaging method, fundus imaging apparatus, and storage medium

ABSTRACT

A fundus imaging apparatus includes: an aberration measurement unit adapted to measure an aberration of reflected light obtained by irradiating an object to be examined with measurement light; an aberration correction unit adapted to correct an aberration of light in accordance with the measured aberration; a control unit adapted to repeatedly control processing of the aberration measurement unit and the aberration correction unit; and a changing unit adapted to change a first function of a predetermined order representing the aberration to a second function including an order higher than the predetermined order in accordance with at least one of a measurement result obtained by the aberration measurement unit and a control result obtained by the control unit. The aberration correction unit corrects an aberration expressed by the second function.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a fundus imaging method, a fundusimaging apparatus, and a storage medium.

2. Description of the Related Art

Recently, as an ophthalmic imaging apparatus, an SLO (Scanning LaserOphthalmoscope) has been developed, which two-dimensionally irradiatesthe fundus with a laser beam, receives reflected light, and images thelight. In addition, as an ophthalmic imaging apparatus, an imagingapparatus using low-coherence light interference has been developed. Theimaging apparatus using lower-coherence light interference is called anOCT (Optical Coherence Tomography), which is used for the purpose ofobtaining a tomogram of the fundus or its neighboring region, inparticular. Various types of OCTs have been developed, including aTD-OCT (Time Domain OCT) and an SD-OCT (Spectral Domain OCT). Ophthalmicimaging apparatuses have recently been increased in resolution withincreasing NA of irradiation lasers.

When, however, imaging the fundus, it is necessary to perform imagingthrough optical tissues of the eye, such as the cornea and thecrystalline lens. For this reason, with an increase in resolution, theaberrations of the cornea and crystalline lens have started to greatlyinfluence the image quality of captured images.

Under the circumstances, studies have been made on an AO (AdaptiveOptics)-SLO and AO-OCT, in which an optical system incorporates an AOfunction for measuring the aberrations of the eye and correcting them.For example, non-patent literature 1 (Y. Zhang et al, Optics Express,Vol. 14, No. 10, 15 May 2006) discloses an example of an AO-OCT. SuchAO-SLO and AO-OCT measure the wavefront of the eye by the Shack-Hartmannwavefront sensor system. The Shack-Hartmann wavefront sensor system isdesigned to measure the wavefront of the eye by applying measurementlight to the eye and making a CCD camera receive the reflected lightthrough a microlens array. An AO-SLO or AO-OCT can performhigh-resolution imaging by driving a deformable mirror and a spatialphase modulator so as to correct a measured wavefront and imaging thefundus through them.

Most of the aberrations of the eye are lower-order aberrations, such asmyopia, hyperopia, and astigmatism. However, the aberrations alsoinclude higher-order aberrations due to the fine recesses andprojections on the optical system of the eye and the disturbance of atear film. When the aberrations of the eye are to be expressed by aZernike function system, most of the Zernike functions expressing theaberrations are Zernike second-order functions expressing myopia,hyperopia, and astigmatism. These functions slightly include Zernikethird-order functions and Zernike fourth-order functions, and moreslightly include higher-order functions such as Zernike fifth-orderfunctions and Zernike sixth-order functions.

In general, the adaptive optics (AO) used in an ophthalmic apparatusmodels an aberration measured by the wavefront sensor with a functionsuch as a Zernike function and calculates a correction amount for awavefront correction unit by using the function. The amountquantitatively obtained by modeling an aberration with a function willbe referred to as an amount of aberration. In addition, awavefront-correction value with which the wavefront correction unitcorrects an aberration by using the function will be referred to as acorrection amount. In order to correct a complex shape, it is necessaryto model an aberration with a function having many orders, calculate acorrection amount, and control the wavefront correction unit.

If, however, a correction amount is calculated by modeling an aberrationwith a function having many orders, the calculation load becomes veryheavy, and the calculation time increases, thus posing a seriousproblem. For the aberrations of the eye, in particular, it is veryimportant to increase the processing speed because the state of tear andthe state of dioptic adjustment always change and aberration correctionneeds to be repeated fast for the acquisition of a tomogram.

SUMMARY OF THE INVENTION

The present invention has been made in consideration of the aboveproblems and provides a fundus imaging technique capable of performingcomputation processing for aberration correction at high speed.

According to one aspect of the present invention, there is provided afundus imaging method for a fundus imaging apparatus including anaberration measurement unit adapted to measure an aberration ofreflected light obtained by irradiating an object to be examined withmeasurement light, an aberration correction unit adapted to correct anaberration of light in accordance with the measured aberration, and acontrol unit adapted to repeatedly control processing of the aberrationmeasurement unit and the aberration correction unit, the methodcomprising: a changing step of changing a first function of apredetermined order representing the aberration to a second functionincluding an order higher than the predetermined order in accordancewith at least one of a measurement result obtained by the aberrationmeasurement unit and a control result obtained by the control unit; andan aberration correction step of correcting an aberration expressed bythe second function.

According to another aspect of the present invention, there is provideda fundus imaging apparatus comprising: an aberration measurement unitadapted to measure an aberration of reflected light obtained byirradiating an object to be examined with measurement light; anaberration correction unit adapted to correct an aberration of light inaccordance with the measured aberration; a control unit adapted torepeatedly control processing of the aberration measurement unit and theaberration correction unit; and a changing unit adapted to change afirst function of a predetermined order representing the aberration to asecond function including an order higher than the predetermined orderin accordance with at least one of a measurement result obtained by theaberration measurement unit and a control result obtained by the controlunit, wherein the aberration correction unit corrects an aberrationexpressed by the second function.

According to the present invention, it is possible to performcomputation processing for aberration correction at high speed.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments (with reference to theattached drawings).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing an example of the arrangement of a fundusimaging apparatus based on SLO including an adaptive-optics systemaccording to the first embodiment;

FIG. 2 is a schematic view showing an example of a wavefront correctiondevice in the first embodiment;

FIG. 3 is a view showing another example of the arrangement of thewavefront correction device;

FIGS. 4A and 4B are schematic views showing the arrangement of aShack-Hartmann wavefront sensor;

FIG. 5 is a schematic view showing a state in which light beams formeasuring a wavefront are focused on a CCD sensor;

FIG. 6 is a schematic view showing a case in which a wavefront having aspherical aberration is measured;

FIG. 7 is a flowchart showing control steps in the fundus imagingapparatus according to the first embodiment;

FIG. 8 is a flowchart showing control steps in a fundus imagingapparatus according to the second embodiment;

FIG. 9 is a flowchart showing control steps in a fundus imagingapparatus according to the third embodiment;

FIG. 10 is a view showing an example of the arrangement of a fundusimaging apparatus based on SLO including an adaptive-optics systemaccording to the fourth embodiment; and

FIG. 11 is a flowchart showing control steps in the fundus imagingapparatus according to the fourth embodiment.

DESCRIPTION OF THE EMBODIMENTS

The embodiments of the present invention will be exemplarily describedin detail below with reference to the accompanying drawings. Theconstituent elements described in these embodiments are merely examples,and the technical range of the present invention is defined by theappended claims, but is not limited to each embodiment described below.

First Embodiment

The arrangement of a fundus imaging apparatus according to the firstembodiment of the present invention will be described with reference toFIG. 1. Note that this embodiment will exemplify a case in which anobject to be examined as a measurement target is an eye, aberrationsoccurring in the eye are corrected by the adaptive-optics system, andthe fundus is imaged.

Referring to FIG. 1, this apparatus uses an SLD (Super LuminescentDiode) light source having a wavelength of 840 nm as a light source 101.Although the wavelength of the light source 101 is not specificallylimited, a wavelength of about 800 nm to 1,500 nm is used for fundusimaging to reduce the glare of light at an object and maintainresolution. Although this embodiment uses the SLD light source, it ispossible to use a laser or the like. Although the embodiment uses thesame light source for fundus imaging and wavefront measurement, it ispossible to use different light sources to form an arrangement formultiplexing light midway along the optical path.

The light emitted from the light source 101 passes through a single-modeoptical fiber 102, and is applied as a parallel light beam (measurementlight 105) through a collimator 103. The applied measurement light 105is transmitted through a light splitting unit 104 formed from a beamsplitter, and is guided to an adaptive-optics system. Theadaptive-optics system includes a light splitting unit 106, a wavefrontsensor 115, a wavefront correction device 108, and reflecting mirrors107-1 to 107-4 for guiding light to them. This embodiment uses a beamsplitter as the light splitting unit 106. The reflecting mirrors 107-1to 107-4 are arranged to make at least the pupil of an eye 111, thewavefront sensor 115, and the wavefront correction device 108 have anoptically conjugated relation.

The measurement light 105 transmitted through the light splitting unit106 is reflected by the reflecting mirrors 107-1 and 107-2 and entersthe wavefront correction device 108. The measurement light 105 reflectedby the wavefront correction device 108 emerges to the reflecting mirror107-3. This embodiment uses a spatial phase modulator (reflectiveliquid-crystal optical modulator) using a liquid-crystal device as thewavefront correction device 108. FIG. 2 is a schematic view of areflective liquid-crystal optical modulator. The reflectiveliquid-crystal optical modulator has a structure having liquid-crystalmolecules sealed in the space defined by a base portion 122 and a cover123. The base portion 122 has a plurality of pixel electrodes 124, andthe cover 123 has a transparent opposite electrode (not shown). In theabsence of a voltage between the electrodes, the liquid-crystalmolecules exhibit an alignment state like that indicated by “125-1”.When a voltage is applied between the electrodes, the liquid-crystalmolecules make a transition to an alignment state like that indicated by“125-2”, and change in refractive index with respect to incident light.It is possible to perform spatial phase modulation by controlling thevoltage to each pixel electrode 124 so as to change its refractiveindex. When, for example, incident light 126 enters the reflectiveliquid-crystal optical modulator, light passing through theliquid-crystal molecules 125-2 shows a phase lag relative to lightpassing through the liquid-crystal molecules 125-1. As a result, awavefront like that indicated by a broken line 127 in FIG. 2 is formed.In general, the reflective liquid-crystal optical modulator isconstituted by several ten thousand to several hundred thousand pixels.In addition, the reflective liquid-crystal optical modulator may includea polarizing unit for adjusting the polarization of incident light so asto have a polarization characteristic.

A deformable mirror is available as another example of the wavefrontcorrection device 108. The deformable mirror can locally change thereflecting direction of light. Various types of deformable mirrors areavailable. The deformable mirror is formed as, for example, a devicehaving the section shown in FIG. 3, which includes a mirror surface 129in the form of a deformable film that reflects incident light, a baseportion 128, actuators 130 sandwiched between the mirror surface 129 andthe base portion 128, and a support portion (not shown) that supportsthe mirror surface 129 with respect to its surrounding. The principle ofoperation of the actuators 130 includes using electrostatic force,magnetic force, or the piezoelectric effect. The actuators 130 differ inarrangement depending on the principle of operation. The actuators 130are two-dimensionally arranged in a plurality of arrays on the baseportion 128 within an x-y plane. Selectively driving the actuators 130can freely deform the mirror surface 129 in the z direction in FIG. 3.In general, a deformable mirror is constituted by several ten to severalhundred of actuators.

Referring to FIG. 1, a scanning optical system 109 one-dimensionally ortwo-dimensionally scans the light reflected by the reflecting mirrors107-3 and 107-4. In this embodiment, the scanning optical system 109includes two Galvanometer scanners respectively used for a main scanningoperation (the horizontal direction of the fundus) and a sub-scanningoperation (the vertical direction of the fundus). For a faster imagingoperation, resonant scanners may be used for the main scanning operationof the scanning optical system 109. In order to set the respectivescanners in the scanning optical system 109 in an optically conjugatedstate, the scanning optical system 109 may have an arrangement usingoptical elements, such as mirrors and lenses, between the respectivescanners.

The measurement light 105 scanned by the scanning optical system 109passes through eyepiece lenses 110-1 and 110-2 and enters the eye 111.The measurement light 105 entering the eye 111 is reflected or scatteredby the fundus. Adjusting the positions of the eyepiece lenses 110-1 and110-2 can optimally irradiate the eye 111 with light in accordance withthe diopter of the eye 111. In this case, the eyepiece lenses are usedfor the eyepiece portion, but it is possible to use spherical mirrorsand the like.

The light reflected or scattered by the retina of the eye 111 reverselypropagates along the path of the incident light. The light splittingunit 106 then reflects part of the light to the wavefront sensor 115,which in turn uses the light to measure the wavefront of the light beam.

This embodiment uses a Shack-Hartmann wavefront sensor as the wavefrontsensor 115. FIGS. 4A and 4B are schematic views showing theShack-Hartmann wavefront sensor. Reference numeral 131 denotes a lightbeam whose wavefront is to be measured, which passes through a microlensarray 132 and is focused on a focal plane 134 on a CCD sensor 133. FIG.4B is a sectional view taken along a line A-A′ in FIG. 4A. The microlensarray 132 is constituted by a plurality of microlenses 135. The lightbeam 131 is focused on the CCD sensor 133 through each microlens 135. Asa result, the light beam 131 is split into spots corresponding to thenumber of the microlenses 135 and focused. FIG. 5 shows how light isfocused on the CCD sensor 133. The light beam passing through eachmicrolens is focused into a spot 136. The wavefront sensor thencalculates the wavefront of the light beams applied from the positionsof the spots 136. For example, FIG. 6 is a schematic view showing a casein which a wavefront having a spherical aberration is measured. Thelight beams 131 are formed by a wavefront like that indicated by abroken line 137. The microlens array 132 focuses the light beams 131 atpositions in directions locally perpendicular to the wavefront. FIG. 6shows a focused state on the CCD sensor 133 in this case. Since thelight beam 131 has a spherical aberration, the spot 136 is focused in astate offset to the middle portion. Calculating this position can obtainthe wavefront of the light beam 131. This embodiment uses theShack-Hartmann wavefront sensor for the wavefront sensor 115. However,the gist of the present invention is not limited to this. For example,it is possible to use another type of wavefront measuring unit, such asa curvature sensor or a method of obtaining a wavefront from a formedpoint image by inverse calculation.

Referring back to FIG. 1, the reflected light transmitted through thelight splitting unit 106 in FIG. 1 is partly reflected by the lightsplitting unit 104 and is guided to a light intensity sensor 114 througha collimator 112 and an optical fiber 113. The light intensity sensor114 then converts the light into an electrical signal. The CP 117 formsthe signal into a fundus image and displays it on a display unit 118.

The wavefront sensor 115 is connected to an adaptive-optics control unit116 to transfer the received wavefront to the adaptive-optics controlunit 116. The wavefront correction device 108 is also connected to theadaptive-optics control unit 116 to perform modulation instructed fromthe adaptive-optics control unit 116. The adaptive-optics control unit116 calculates a modulation amount (correction amount) for correctiontoward a wavefront with no aberration based on the wavefront acquiredfrom the measurement result obtained by the wavefront sensor 115. Theadaptive-optics control unit 116 instructs the wavefront correctiondevice 108 to perform modulation based on the modulation amount(correction amount) calculated by the adaptive-optics control unit 116.The adaptive-optics control unit 116 repeatedly instructs the wavefrontcorrection device 108 based on the measurement of a wavefront by thewavefront sensor 115 and the modulation amount (correction amount)calculated based on the measurement result, thus always performingfeedback control to obtain an optimal wavefront. In this embodiment, theadaptive-optics control unit 116 models the measured wavefront into aZernike function and calculates coefficients applied to the respectiveorders of the function. The adaptive-optics control unit 116 thencalculates a modulation amount for the wavefront correction device 108based on the coefficients. When calculating a modulation amount, theadaptive-optics control unit 116 multiplies the coefficients of all theZernike orders by reference modulation amounts that allow the wavefrontcorrection device 108 to form the shapes of the respective Zernikeorders, and adds up all the products, thereby obtaining a finalmodulation amount. This embodiment uses a reflective liquid-crystaloptical modulator with a pixel count of 600×600 as the wavefrontcorrection device 108, and hence calculates a modulation amount for eachof 360,000 pixels in accordance with the above-described calculationmethod. When, for example, performing a calculation using thecoefficients of the first to fourth orders of a Zernike function, theembodiment multiplies 14 coefficients, namely Z1−1, Z1+1, Z2−2, Z2−0,Z2+2, Z3−3, Z3−1, Z3+1, Z3+3, Z4−4, Z4−2, Z4−0, Z4+2, and Z4+4 byreference modulation amounts for each of the 360,000 pixels. Whenperforming a calculation using the coefficients of the first to sixthorders of the Zernike function, the embodiment multiplies 27coefficients, namely Z1−1, Z1+1, Z2−2, Z2−0, Z2+2, Z3−3, Z3−1, Z3+1,Z3+3, Z4−4, Z4−2, Z4−0, Z4+2, Z4+4, Z5−5, Z5−3, Z5−1, Z5+1, Z5+3, Z5+5,Z6−6, Z6−4, Z6−2, Z6−0, Z6+2, Z6+4, and Z6+6 by reference modulationamounts for each of the 360,000 pixels. Since the eye to be examined isincluded in part of the optical system, the state of optical system isuncertain. For this reason, it is generally difficult to reach awavefront with low aberration by one aberration measurement andcorrection. Therefore, aberration measurement and correction arerepeated to correct the aberration to an extent to allow an imagingoperation.

As described above, most of the aberrations of the eye are lower-orderaberrations. For this reason, this embodiment performsaberration-correction feedback (first aberration-correction feedback) athigh speed by using the coefficients of the lower orders, namely thefirst to fourth orders, of the Zernike function at the beginning of thestart of correction. At a midway stage of aberration correction, theembodiment performs more precise aberration-correction feedback (secondaberration-correction feedback) by using the coefficients of the higherorders, namely the first to sixth orders, of the Zernike function.Performing such control makes it possible to perform aberrationcorrection up to a low aberration state at a relatively high speed. Thiscan shorten the time to the start of an imaging operation.

Processing in the fundus imaging apparatus according to this embodimentwill be described with reference to the flowchart of FIG. 7. In stepS101, the fundus imaging apparatus starts a control operation. In stepS102, the apparatus sets coefficients up to the Nth order (N is anatural number (ditto for the following)) to express an aberration by aZernike function, which is a polynomial function. In this embodiment,the apparatus sets the coefficients of the Zernike function to be usedfor aberration correction to the coefficients of the lower orders,namely the first to fourth orders. The apparatus then executes the basicprocedure in the adaptive optics from step S103 to step S109 (to bedescribed below). The following is an outline of the basic procedure inthe adaptive optics. In step S103, the wavefront sensor 115 measures anaberration. In step S108, the adaptive-optics control unit 116calculates a correction amount based on the measurement result. In stepS109, the apparatus repeatedly drives the wavefront correction device108 under the control of the adaptive-optics control unit 116.

The contents of each step in the adaptive optics will be described next.In step S103, the wavefront sensor 115 measures an aberration andobtains the amount of aberration. In this embodiment, the amount ofaberration indicates the total amount of a wavefront disturbanceobtained from the obtained aberration. However, this amount mayindicate, for example, the total amount of deviation from a referencewavefront (flat wavefront). In step S104 (first determination step), theadaptive-optics control unit 116 determines whether the amount ofaberration obtained in step S103 is smaller than the predetermined firstreference value (reference 1). The first reference value (reference 1)may be a value unique to the fundus imaging apparatus or may be set bythe operator. If the amount of aberration is smaller than the firstreference value (reference 1) (YES in step S104), the process advancesto step S110. If the amount of aberration is equal to or more than thefirst reference value (reference 1) (NO in step S104), the processadvances to step S105 to execute the processing in step S105 and thesubsequent steps.

In step S105 (second determination step), the adaptive-optics controlunit 116 obtains a change in the amount of aberration from thedifference between the amount of aberration that has already beenmeasured (for example, the previously measured amount of aberration) andthe amount of aberration obtained by the current measurement. Theadaptive-optics control unit 116 then determines whether the change inthe amount of aberration is smaller than the second reference value(reference 2). It is possible to use, as the second reference value(reference 2), for example, a value representing the rate of change inthe difference between the previous amount of aberration and the currentamount of aberration or a value determined by the operator in advance.If the change in the amount of aberration is smaller than the secondreference value (reference 2) (YES in step S105), the adaptive-opticscontrol unit 116 determines that it is highly possible that the use ofthe coefficients of the orders currently used may not allow sufficientcorrection. The process therefore advances to step S107. In step S107,the adaptive-optics control unit 116 changes the Nth-order polynomialfunction to an Mth-order (M is a natural number satisfying the relationof M>N (ditto for the following)) function including orders higher thanthe Nth order in accordance with the change between the measured amountof aberration and the amount of aberration measured after correction.The adaptive-optics control unit 116 sets the coefficients of theZernike function to be used for aberration correction to thecoefficients of a function including the higher orders, namely the firstto sixth orders. The process then returns to step S103. This embodimentexemplifies the setting of the coefficients of the first to sixth ordersas the coefficients of the higher orders. If, however, the coefficientsof the first to fourth orders are set as the coefficients of lowerorders, the coefficients of higher orders are not limited to thecoefficients of the first to sixth orders. For example, it is possibleto set the coefficients of the first to fifth orders including thecoefficients of the first to fourth orders or the coefficients of thefirst to sixth orders. In addition, the coefficients of the sixth orderare not limited as the upper-limit coefficients, and the coefficients tobe used can include those of higher orders.

In step S103 and the subsequent steps, the adaptive-optics control unit116 performs similar aberration-correction processing by using thefunction including the coefficients of the higher orders set in stepS107 previously executed.

If the adaptive-optics control unit 116 determines in step S105 that thechange in the amount of aberration is equal to more than the secondreference value (reference 2) (NO in step S105), the unit determinesthat the aberration correction corresponding to the function includingthe orders currently used is not sufficient. The adaptive-optics controlunit 116 therefore advances the process to step S106 to continuecorrection without changing the coefficients of the orders currentlyused.

In step S106 (third determination step), the adaptive-optics controlunit 116 determines whether the repeat count of correction control fromthe start of aberration correction exceeds the third reference value(reference 3). If the repeat count exceeds the third reference value(reference 3) (YES in step S106), the adaptive-optics control unit 116advances the process to step S107. In step S107, the adaptive-opticscontrol unit 116 changes the function (first function) havingpredetermined orders to a function (second function) including ordershigher than the predetermined orders in accordance with the measurementresult obtained by the wavefront sensor 115. In this case, themeasurement result obtained by the wavefront sensor 115 includes atleast one of the amount of aberration measured by the wavefront sensor115 and the change in the amount of aberration obtained from thedifference between the amount of aberration that has already beenmeasured and the amount of aberration measured by the wavefront sensor115. The adaptive-optics control unit 116 changes the coefficients ofthe Zernike function to be used to, for example, the coefficients of afunction having orders including the higher orders, namely the first tosixth orders, and returns the process to step S103. Although the abovedescription has exemplified the control result obtained by theadaptive-optics control unit 116 using the repeat count of correctioncontrol as the third reference in step S106, the gist of the presentinvention is not limited to this. For example, it is possible to obtaina control result by using, as the third reference, a repetition timeindicating the lapse of time from the start of correction controlprocessing. The adaptive-optics control unit 116 includes a timepieceunit (for example, a timer) capable of measuring the lapse of time fromthe start of processing, and determines whether a predeterminedrepetition time has elapsed from the start of correction controlprocessing (S106). After the predetermined repetition time has elapsedfrom the start of the processing (YES in step S106), the processadvances to step S107. In step S107, the adaptive-optics control unit116 executes the processing of changing the function (first function)having predetermined orders to the function (second function) havingorders including orders higher than the predetermined orders inaccordance with the measurement result obtained by the wavefront sensor115.

If the adaptive-optics control unit 116 determines in step S106 that therepeat count of correction control from the start of aberrationcorrection is equal to or smaller than the third reference value(reference 3) (NO in step S106), the process advances to step S108. Instep S108, the adaptive-optics control unit 116 calculates a correctionamount for the correction of the aberration, which is expressed by thechanged function. In step S109, the apparatus drives the wavefrontcorrection device 108 under the control of the adaptive-optics controlunit 116, and executes correction processing for the correction of theaberration expressed by the changed function. The process then returnsto step S103. The apparatus repeats the processing from step S103 tostep S109 until the adaptive-optics control unit 116 determines in stepS104 that the amount of aberration is smaller than the first referencevalue (reference 1).

The fundus imaging apparatus images the fundus of the eye in step S110,and determines in step S111 whether to terminate the processing. If notermination request is input (NO in step S111), the process returns tostep S103 to perform the processing in the adaptive optics from stepS103 to step S109 and perform imaging in step S110. In the example ofthe control operation of the fundus imaging apparatus shown in FIG. 7,the apparatus sequentially performs the process for imaging and theprocess for aberration correction. It is however possible toconcurrently perform both the processes. If the apparatus confirms atermination request in step S111 (YES in step S111), the apparatusterminates the control operation in step S112.

In aberration-correction processing, since the calculation time for amodulation amount (correction amount) occupies a very high ratio of thetotal processing time, a reduction in the calculation amount is veryeffective in speeding up the processing. The time taken to calculate amodulation amount by using the coefficients of the first to fourthorders of the Zernike function differs from that taken to calculate amodulation amount by using the coefficients of the first to sixth ordersof the Zernike function by almost two times. Performing correction byusing the coefficients of the first to fourth orders of the Zernikefunction at the start of correction as in this embodiment will greatlyspeed up the processing. If most of measured aberrations are aberrationscorresponding to the first to fourth orders of the Zernike function, itis possible to sufficiently correct the aberrations by using thecoefficients of the first to fourth orders of the Zernike function. Theprocess therefore advances from step S104 to step S110 to allow imagingto quickly start. If sufficient correction could not be performed byusing the coefficients of the first to fourth orders of the Zernikefunction, this apparatus processes only the residual aberrations, whichcould not be corrected, by using the coefficients of orders includingthe higher orders of the Zernike function, and hence can reach a readystate for imaging with a small correction count. Performing suchprocessing can correct aberrations that allow imaging faster thanrepeating control using coefficients including those of the higherorders from the start of processing. This embodiment uses Zernikefunctions to model aberrations. However, the same applies to a case inwhich the apparatus uses other function systems.

This embodiment allows the use of a proper correction method inaccordance with a correction state, and hence can speed upaberration-correction processing and implement high-speed imaging.

Second Embodiment

Processing in a fundus imaging apparatus according to the secondembodiment will be described with reference to the flowchart of FIG. 8.The basic apparatus arrangement is the same as that of the firstembodiment. This embodiment has a feature of performing aberrationcorrection by using coefficients corresponding to large aberrations ofthe aberrations of the eye to be examined as dominant coefficients inaberration correction. The fundus imaging apparatus starts a controloperation in step S201. In step S213, the apparatus measures anaberration before correction. An adaptive-optics control unit 116 modelsthe measured aberration by using the first to sixth orders of a Zernikefunction, and checks a term (order) of the modeled terms (orders) whichhas large coefficients. In step S202, the apparatus sets coefficientsfor the term (order) having large values checked in step S213 ascoefficients to be used for correction control. The apparatus thenexecutes a basic procedure in the adaptive optics.

The following is an outline of the basic procedure in the adaptiveoptics. In step S203, a wavefront sensor 115 measures an aberration. Instep S208, the adaptive-optics control unit 116 calculates a correctionamount based on the measurement result. In step S209, the apparatusrepeatedly drives a wavefront correction device 108 under the control ofthe adaptive-optics control unit 116. The contents of each step in theadaptive optics will be described next. In step S203, the wavefrontsensor 115 measures an aberration and obtains an amount of aberration.

In step S204, the adaptive-optics control unit 116 determines whetherthe amount of aberration obtained in step S203 is smaller than thepredetermined first reference value (reference 1). The first referencevalue (reference 1) may be a value unique to the fundus imagingapparatus or may be set by the operator. If the amount of aberration issmaller than the first reference value (reference 1) (YES in step S204),the process advances to step S110. If the amount of aberration is equalto or more than the first reference value (reference 1) (NO in stepS204), the process advances to step S205 to execute the processing instep S205 and the subsequent steps.

In step S205, the adaptive-optics control unit 116 then determineswhether a change in the amount of aberration exceeds the secondreference value (reference 2). If the change in the amount of aberrationis smaller than the second reference value (reference 2) (YES in stepS205), the adaptive-optics control unit 116 determines that it is highlypossible that the use of the coefficient of the orders currently usedmay not allow sufficient correction. The process therefore advances tostep S207. In step S207, the adaptive-optics control unit 116 changesthe coefficients of the Zernike function to be used to the coefficientsof a function having higher orders, namely the first to sixth orders (afunction having the correction coefficients of higher orders), andreturns the process to step S203. In step S203 and the subsequent steps,the adaptive-optics control unit 116 performs similaraberration-correction processing by using the function having the higherorders changed in step S207.

If the adaptive-optics control unit 116 determines in step S205 that thechange in the amount of aberration is equal to or more than the secondreference value (reference 2) (NO in step S205), the unit determinesthat the aberration correction corresponding to the coefficients of thefunction currently used is not sufficient. The adaptive-optics controlunit 116 therefore advances the process to step S206 to continuecorrection without changing the coefficients of the orders currentlyused.

In step S206, the adaptive-optics control unit 116 determines whetherthe repeat count of correction control from the start of aberrationcorrection exceeds the third reference value (reference 3). In thiscase, the third reference value (reference 3) is a predeterminedconstant N (N is a natural number equal to or more than 2). If therepeat count exceeds the third reference value (reference 3) (YES instep S206), the adaptive-optics control unit 116 advances the process tostep S207. In step S207, the adaptive-optics control unit 116 changesthe coefficients of the Zernike function to be used to the coefficientsof orders including the higher orders, namely the first to sixth orders,(correction coefficients of the higher orders), and returns the processto step S203.

If the adaptive-optics control unit 116 determines in step S206 that therepeat count of correction control from the start of aberrationcorrection is equal to or smaller than the third reference value(reference 3) (NO in step S206), the process advances to step S208. Instep S208, the adaptive-optics control unit 116 calculates a correctionamount. In step S209, the apparatus drives the wavefront correctiondevice 108 under the control of the adaptive-optics control unit 116.The process then returns to step S203. The apparatus repeats theprocessing from step S203 to step S209 until the adaptive-optics controlunit 116 determines in step S204 that the amount of aberration issmaller than the first reference value (reference 1).

The fundus imaging apparatus images the fundus of the eye in step S210,and determines in step S211 whether to terminate the processing. If notermination request is input (NO in step S211), the process returns tostep S203 to perform the processing in the adaptive optics from stepS203 to step S209 and perform imaging in step S210. In the example ofthe control operation of the fundus imaging apparatus shown in FIG. 8,the apparatus sequentially performs the process for imaging and theprocess for aberration correction. It is however possible toconcurrently perform both the processes. If the apparatus confirms atermination request in step S211 (YES in step S211), the apparatusterminates the control operation in step S212.

Limiting orders to be corrected at the start of correction to portionshaving large aberrations can greatly speed up the processing. Sinceorders having large aberrations are correction targets, it is highlypossible to reach an aberration state allowing imaging by correctingthese orders. This makes it possible to quickly start imaging. Even ifthe aberrations cannot be corrected with the initial settings, since theapparatus corrects only the residual aberrations which could not becorrected, it is therefore possible to reach an aberration stateallowing imaging more quickly than repeating control using thecoefficients of orders including higher orders from the beginning of theprocessing. This embodiment uses Zernike functions to model aberrations.The same applied to a case in which the apparatus uses other functionsystems.

This embodiment allows to use a proper correction method in accordancewith the aberration state of the eye to be examined, and hence can speedup aberration-correction processing and implement high-speed imaging.

Third Embodiment

Processing in a fundus imaging apparatus according to the thirdembodiment will be described with reference to the flowchart of FIG. 9.The basic apparatus arrangement is the same as that of the firstembodiment. This embodiment has a feature of speeding up correctioncontrol to improve maintenance accuracy in an arrangement formaintaining a low aberration state after the completion of correction upto aberrations allowing fundus imaging.

In step S301, the fundus imaging apparatus starts a control operation.In step S302, the apparatus sets coefficients up to the Nth order toexpress an aberration by a Zernike function, which is a polynomialfunction. In this embodiment, the apparatus sets the coefficients of theZernike function to be used for aberration correction to thecoefficients of the lower orders, namely the first to fourth orders. Theapparatus then executes the basic procedure in the adaptive optics fromstep S303 to step S309 (to be described below). The following is anoutline of the basic procedure in the adaptive optics. In step S303, awavefront sensor 115 measures an aberration. In step S308, anadaptive-optics control unit 116 calculates a correction amount based onthe measurement result. In step S309, the apparatus repeatedly drives awavefront correction device 108 under the control of the adaptive-opticscontrol unit 116.

The contents of each step in the adaptive optics will be described next.In step S303, the wavefront sensor 115 measures an aberration andobtains the amount of aberration.

In step S304, the adaptive-optics control unit 116 determines whetherthe amount of aberration obtained in step S303 is smaller than thepredetermined first reference value (reference 1). The first referencevalue (reference 1) may be a value unique to the fundus imagingapparatus or may be set by the operator. If the amount of aberration issmaller than the first reference value (reference 1) (YES in step S304),the process advances to step S310. If the amount of aberration is equalto or more than the first reference value (reference 1) (NO in stepS304), the process advances to step S305 to execute the processing instep S305 and the subsequent steps. As in the processing in steps S105and S106 in the first embodiment, the adaptive-optics control unit 116determines the rate of change in the amount of aberration and the repeatcount in steps S305 and S306. The adaptive-optics control unit 116 thencompares each value with a corresponding reference value. If theycoincide with each other (YES in step S305 and YES in step S306), theprocess advances to step S307. In step S307, the adaptive-optics controlunit 116 changes the Nth-order (N is a natural number) function to anMth-order (M is a natural number satisfying the relation of M>N)function including orders higher than the Nth order in accordance withthe change between the measured amount of aberration and the amount ofaberration measured after correction. The adaptive-optics control unit116 sets the coefficients of the Zernike function to be used to thecoefficients of a function including the higher orders, namely the firstto sixth orders. The process then returns to step S103. This embodimentexemplifies the setting of the coefficients of the first to sixth ordersas the coefficients of the higher orders. If, however, the coefficientsof the first to fourth orders are set as the coefficients of lowerorders, the coefficients of higher orders are not limited to thecoefficients of the first to sixth orders. For example, it is possibleto set the coefficients of the first to fifth orders including thecoefficients of the first to fourth orders or the coefficients of thefirst to sixth orders. In addition, the coefficient of the sixth orderare not limited as the upper-limit coefficients, and the coefficients tobe used can include those of higher orders.

If the adaptive-optics control unit 116 determines upon comparison withthe respective reference values in steps S305 and S306 that therespective values do not coincide with the set conditions (NO in stepS305 and NO in step S306), the process advances to step S308. In stepS308, the adaptive-optics control unit 116 calculates a correctionamount. In step S309, the apparatus drives the wavefront correctiondevice 108 under the control of the adaptive-optics control unit 116.The process then returns to step S303. The apparatus repeats theprocessing from step S303 to step S309 until the adaptive-optics controlunit 116 determines in step S304 that the amount of aberration issmaller than the first reference value (reference 1).

The fundus imaging apparatus images the fundus of the eye in step S310,and determines in step S311 whether to terminate the processing. If theapparatus confirms a termination request in step S311 (YES in stepS311), the apparatus terminates the control operation in step S312.

If the adaptive-optics control unit 116 determines in step S311 that notermination request has been input (NO in step S311), the processadvances to step S314. In step S314, the adaptive-optics control unit116 selects the coefficients of an order exhibiting a large change(fluctuation) from the measured aberration, and sets the coefficients ofthe order as coefficients to be used for correction. It is possible toselect, as coefficients to be used for correction, any number ofcoefficients exhibiting large fluctuation amounts or the coefficients oforders corresponding to a predetermined fluctuation amount or more. Inmeasuring, for example, the undilated eye, since the refractionadjustment of the eye always fluctuates, it is necessary to follow upthis refracted state in order to perform imaging with high imagequality. In this case, since lower-order aberrations greatly fluctuate,performing aberration correction by using the coefficients of lowerorders improves the follow-up performance. It is known that higher-orderaberrations fluctuate depending on the state of tear fluid. If,therefore, higher-order aberrations greatly fluctuate, aberrationcorrection may be performed by using the coefficients of higher orders.

After setting coefficients, the apparatus performs basic processing ofthe adaptive optics in step S303 and the subsequent steps, and thenperforms the next imaging operation in step S310. In the example of thecontrol operation of the fundus imaging apparatus shown in FIG. 9, theapparatus sequentially performs the process for imaging and the processfor aberration correction. It is however possible to concurrentlyperform both the processes.

This embodiment improves the follow-up performance for aberrationcorrection at the time of continuous imaging operation, and hence cancapture a plurality of high-quality images in a short period of time.

Fourth Embodiment

The arrangement of a fundus imaging apparatus according to the fourthembodiment will be described with reference to FIG. 10. The basicarrangement shown in FIG. 10 is the same as that of the fundus imagingapparatus of the first embodiment, and the same reference numeralsdenote the same constituent elements. In the fundus imaging apparatusaccording to the fourth embodiment, a wavefront correction device 108 isconstituted by a first wavefront correction device 108-1 and a secondwavefront correction device 108-2. The fundus imaging apparatus includesreflecting mirrors 107-5 and 107-6 for optically coupling the firstwavefront correction device 108-1 to the second wavefront correctiondevice 108-2. In this case, the first wavefront correction device 108-1is designed to mainly correct lower-order aberrations. For example, adeformable mirror including a small number of elements is suitably usedas this device. The second wavefront correction device 108-2 is designedto correct aberrations including higher-order aberrations. A deformablemirror including a large number of elements or a spatial phase modulator(reflective liquid-crystal optical modulator) using a liquid-crystaldevice is suitably used as this device. An adaptive-optics control unit116 repeatedly controls processing performed by a wavefront sensor 115and the first wavefront correction device 108-1 or processing performedby the wavefront sensor 115 and the first and second wavefrontcorrection devices 108-1 and 108-2.

Processing in the fundus imaging apparatus according to the fourthembodiment will be described with reference to the flowchart of FIG. 11.In step S401, the fundus imaging apparatus starts a control operation.In step S402, the apparatus sets coefficients up to the Nth order (N isa natural number) to express an aberration by a Zernike function, whichis a polynomial function. In this embodiment, the apparatus sets theZernike coefficients to be used for aberration correction to thecoefficients of the first and second orders. The apparatus then executesthe basic procedure in the adaptive optics in steps S403 to S409 (to bedescribed below). The following is an outline of the basic procedure inthe adaptive optics. In step S403, the wavefront sensor 115 measures anaberration. In step S408 (first calculation step), the adaptive-opticscontrol unit 116 calculates a correction amount for the first wavefrontcorrection device 108-1 based on the measurement result. Note that instep S408, the adaptive-optics control unit 116 calculates no correctionamount for the second wavefront correction device 108-2. Limiting thetarget for the calculation of a correction amount to the first wavefrontcorrection device 108-1 can reduce the calculation amount and greatlyspeed up the processing. In step S409, the apparatus repeatedly drivesthe first wavefront correction device 108-1 under the control of theadaptive-optics control unit 116.

The contents of each step in the adaptive optics will be described next.In step S403, the wavefront sensor 115 measures an aberration andobtains the amount of aberration. In step S404, the adaptive-opticscontrol unit 116 determines whether the amount of aberration obtained instep S403 is smaller than the predetermined first reference value(reference 1). The first reference value (reference 1) may be a valueunique to the fundus imaging apparatus or may be set by the operator. Ifthe amount of aberration is smaller than the first reference value(reference 1) (YES in step S404), the process advances to step S410. Ifthe amount of aberration is equal to or more than the first referencevalue (reference 1) (NO in step S404), the process advances to step S405to execute the processing in step S405 and the subsequent steps.

As in the processing in steps S105 and S106 in the first embodiment, theadaptive-optics control unit 116 determines the rate of change in theamount of aberration and the repeat count in steps S405 and S406. Theadaptive-optics control unit 116 then compares each value with acorresponding reference value. If they coincide with each other (YES instep S405 and YES in step S406), the process advances to step S407. Instep S407, the adaptive-optics control unit 116 changes the Nth-order (Nis a natural number) function to an Mth-order (M is a natural numbersatisfying the relation of M>N) function including orders higher thanthe Nth order in accordance with the change between the measured amountof aberration and the amount of aberration measured after correction.The adaptive-optics control unit 116 sets the coefficients of theZernike function to be used to the coefficients of a function includingthe higher orders, namely the first to sixth orders. This embodimentexemplifies the setting of the coefficients of the first to sixth ordersas the coefficients of the higher orders. If, however, the coefficientsof the first and second orders are set as the coefficients of lowerorders, the coefficients of higher orders are not limited to thecoefficients of the first to sixth orders. For example, it is possibleto set the coefficients of the first to third orders including thecoefficients of the first and second orders, the coefficients of thefirst to fourth orders, and the coefficients of the first to fifthorders or the first to sixth orders. In addition, the coefficient of thesixth order are not limited as the upper-limit coefficients, and thecoefficients to be used can include those of higher orders. The Mthorder as a higher order need not always include all the coefficients ofthe Nth order as a lower order. If, for example, the coefficients of thefirst and second orders are set as the coefficients of the Nth order, itis possible to define the coefficients of a function representing anaberration by setting coefficients so as to include the coefficients ofthe second order as some of the coefficients of the Nth order and thecoefficients of orders higher than the Nth order (for example, the thirdand fourth orders).

If the adaptive-optics control unit 116 determines upon comparison withthe respective reference values in steps S405 and S406 that therespective values do not coincide with the set conditions (NO in stepS405 and NO in step S406), the adaptive-optics control unit 116determines not to change the function (first function) represented by apolynomial having predetermined orders (for example, N orders).

The process advances to step S408. In step S408 (third calculationstep), the adaptive-optics control unit 116 calculates a correctionamount for the first wavefront correction device 108-1. In step S409(third aberration correction step), the apparatus drives the firstwavefront correction device 108-1 under the control of theadaptive-optics control unit 116 to correct an aberration ofpredetermined orders (for example, N orders) based on the correctionamount calculated in step S408. The process then returns to step S403.The apparatus repeats the processing from step S403 to step S409 untilthe adaptive-optics control unit 116 determines in step S404 that theamount of aberration is smaller than the first reference value(reference 1).

The fundus imaging apparatus images the fundus of the eye in step S410,and terminates the processing in step S411. As in step S111 in the firstembodiment, the apparatus can check the termination of imaging beforestep S412, and perform continuous imaging.

In step S407, upon changing the Zernike coefficients, the apparatuschanges the function to be used to a function (second function)including orders (for example, M orders (M is a natural numbersatisfying the relation of M>N)) higher than the predetermined orders(for example, N orders) of the function (first function). The processthen advances to step S415. Although the processing from step S415 tostep S420 is the same as that in the basic procedure in the adaptiveoptics described above, they differ in that both the first wavefrontcorrection device 108-1 and the second wavefront correction device 108-2are targets for wavefront correction.

In step S415, the wavefront sensor 115 measures an aberration andobtains the amount of aberration. In step S416, the adaptive-opticscontrol unit 116 determines whether the amount of aberration obtained instep S415 is smaller than the predetermined first reference value(reference 1). If the amount of aberration is smaller than the firstreference value (reference 1) (YES in step S416), the process advancesto step S410. Upon imaging the fundus in step S410, the apparatusterminates the processing in step S412.

If the apparatus determines in step S416 that the amount of aberrationis equal to or more than the first reference value (reference 1) (NO instep S416), the process advances to step S417 to execute the processingin step S417 and the subsequent steps. In step S417, the apparatusexpresses the measured aberration by a function having coefficients oforders higher than predetermined orders. The adaptive-optics controlunit 116 then calculates a correction amount with which the firstwavefront correction device 108-1 (first aberration correction unit)performs aberration correction of a predetermined order for theaberration expressed by the function. Since the first wavefrontcorrection device 108-1 corrects only a lower-order aberration, theadaptive-optics control unit 116 calculates, in step S417, only acorrection amount for the lower-order aberration of the aberrationsmeasured in step S415.

In step S418 (second calculation step), the apparatus expresses themeasured aberration by a function having coefficients of orders higherthan predetermined orders. The adaptive-optics control unit 116calculates a correction amount with which the second wavefrontcorrection device 108-2 (second aberration correction unit) performsaberration correction of the orders higher than the predetermined ordersfor the aberration expressed by the function. Since the second wavefrontcorrection device 108-2 corrects only a higher-order aberration, theadaptive-optics control unit 116 calculates, in step S418, only acorrection amount for the higher-order aberration of the aberrationsmeasured in step S415.

In step S419 (first aberration correction step), the apparatus drivesthe first wavefront correction device 108-1 under the control of theadaptive-optics control unit 116 to correct an aberration ofpredetermined orders of the aberrations expressed by the functionincluding the coefficients of the changed higher orders. In step S420(second aberration correction step), the apparatus drives the secondwavefront correction device 108-2 to correct an aberration, of theaberrations expressed by the function including the coefficients of thechanged higher orders, which has an order higher than the predeterminedorder.

In the processing in step S415 and the subsequent steps, the apparatusis configured to simultaneously control the two wavefront correctiondevices. However, the apparatus can be configured to perform aberrationcorrection by using only the second wavefront correction device in stepS415 and the subsequent steps. In this case, the second wavefrontcorrection device corrects aberrations including lower- and higher-orderaberrations that could not be sufficiently corrected by the firstwavefront correction device in steps S408 and S409.

According to this embodiment, it is possible to efficiently control aplurality of wavefront correction devices and quickly capturehigh-quality images.

Other Embodiments

Aspects of the present invention can also be realized by a computer of asystem or apparatus (or devices such as a CPU or MPU) that reads out andexecutes a program recorded on a memory device to perform the functionsof the above-described embodiment(s), and by a method, the steps ofwhich are performed by a computer of a system or apparatus by, forexample, reading out and executing a program recorded on a memory deviceto perform the functions of the above-described embodiment(s). For thispurpose, the program is provided to the computer for example via anetwork or from a recording medium of various types serving as thememory device (for example, computer-readable medium).

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2010-283728, filed Dec. 20, 2010, which is hereby incorporated byreference herein in its entirety.

What is claimed is:
 1. A fundus imaging method for a fundus imagingapparatus including (a) an aberration measurement unit configured tomeasure an aberration of reflected light obtained by irradiating anobject to be examined with measurement light, (b) an aberrationcorrection unit configured to correct the aberration of light inaccordance with the measured aberration, and (c) a control unitconfigured to repeatedly control processing of the aberrationmeasurement unit and the aberration correction unit, the methodcomprising: a selecting step of selecting between a first functionincluding a predetermined order representing the aberration and a secondfunction including an order higher than the predetermined order inaccordance with at least one of a measurement result obtained by theaberration measurement unit and a control result obtained by the controlunit, wherein the first function includes coefficients of Nth-order toMth-order representing the aberration (N and M are natural numberssatisfying a relation of M>N), and the second function includescoefficients of Nth-order to Lth-order representing the aberration (L isa natural number satisfying a relation of M<L, M≠L); and an aberrationcorrection step of correcting an aberration expressed by the firstfunction or the second function based on a result of the selecting step.2. The method according to claim 1, wherein the measurement resultobtained by the aberration measurement unit includes at least one of themeasured amount of aberration and a change in an amount of aberrationobtained from a difference between an amount of aberration which hasalready been measured and the measured amount of aberration, and whereinthe control result obtained by the control unit is one of a repeat countof the number of times of processing control by the control unit and arepetition time indicating a lapse of time from a start of theprocessing.
 3. The method according to claim 1, further comprising: afirst determination step of determining whether the amount of aberrationof the reflected light measured by the aberration measurement unit isless than a first reference value, and a second determination step ofdetermining whether the change in the amount of aberration is less thana second reference value, when the amount of aberration is not less thanthe first reference value, wherein if it is determined that the changein the amount of aberration is less than the second reference value, thesecond function is selected in the selecting step.
 4. The methodaccording to claim 3, further comprising a third determination step ofdetermining whether the repeat count of the number of times ofprocessing by the aberration correction unit exceeds a third referencevalue, when the change in the amount of aberration is not less than thesecond reference value, wherein if it is determined that the repeatcount of the number of times of processing by the aberration correctionunit exceeds the third reference value, the second function is selectedin the selecting step.
 5. The method according to claim 1, wherein afterthe processing controlled by the control unit is repeated by apredetermined repeat count or a predetermined repetition time haselapsed from the start of the processing, the second function isselected in accordance with the measurement result obtained by theaberration measurement unit in the selecting step.
 6. The methodaccording to claim 1, further comprising a calculation step ofcalculating a correction amount for correction of the aberrationexpressed by the function selected in the selecting step.
 7. The methodaccording to claim 1, wherein the aberration correction unit includes afirst aberration correction unit and a second aberration correctionunit, and the control unit repeatedly controls one of processing by theaberration measurement unit and the first aberration correction unit andprocessing of the aberration measurement unit, the first aberrationcorrection unit, and the second aberration correction unit, and whereinthe aberration correction step includes (a) a first aberrationcorrection step of causing the first aberration correction unit tocorrect an aberration, of aberrations expressed by the second functionselected in the selecting step, which has the predetermined order, ifthe second function is selected in the selecting step, (b) a secondaberration correction step of causing the second aberration correctionunit to correct an aberration, of aberrations expressed by the secondfunction selected in the selecting step, which has an order higher thanthe predetermined order, and (c) a third aberration correction step ofcausing the first aberration correction unit configured to correct anaberration of the predetermined order to correct an aberration expressedby the first function including the predetermined order.
 8. The methodaccording to claim 7, wherein the first aberration correction stepincludes a first calculation step of calculating a correction amountwith which the first aberration correction unit corrects an aberrationexpressed by the second function selected in the selecting step, whereinthe second aberration correction step includes a second calculation stepof calculating a correction amount with which the second aberrationcorrection unit corrects an aberration expressed by the second functionselected in the selecting step, and wherein the third aberrationcorrection step includes a third calculation step of calculating acorrection amount with which the first aberration correction unitcorrects an aberration expressed by the first function including thepredetermined order.
 9. The method according to claim 1, wherein thefunction is a Zernike function.
 10. A fundus imaging apparatuscomprising: an aberration measurement unit configured to measure anaberration of reflected light obtained by irradiating an object to beexamined with measurement light; an aberration correction unitconfigured to correct an aberration of light in accordance with themeasured aberration; a control unit configured to repeatedly controlprocessing of said aberration measurement unit and said aberrationcorrection unit; and a selecting unit configured to select between afirst function including a predetermined order representing theaberration and a second function including an order higher than thepredetermined order in accordance with at least one of a measurementresult obtained by said aberration measurement unit and a control resultobtained by said control unit, wherein the first function includescoefficients of Nth-order to Mth-order representing the aberration (Nand M are natural numbers satisfying a relation of M>N), and the secondfunction includes coefficients of Nth-order to Lth-order representingthe aberration (L is a natural number satisfying a relation of M<L,M≠L), and wherein said aberration correction unit corrects an aberrationexpressed by the first function or the second function.
 11. Theapparatus according to claim 10, wherein the measurement result obtainedby said aberration measurement unit includes at least one of themeasured amount of aberration and a change in an amount of aberrationobtained from a difference between an amount of aberration which hasalready been measured and the measured amount of aberration, and whereinthe control result obtained by said control unit is one of a repeatcount of the number of times said control unit performs processingcontrol and a repetition time indicating a lapse of time from a start ofthe processing.
 12. The apparatus according to claim 10, furthercomprising: a first determination unit configured to determine whetheran amount of aberration of the reflected light measured by saidaberration measurement unit is less than a first reference value, and asecond determination unit configured to determine whether the change inthe amount of aberration is less than a second reference value, when theamount of aberration is not less than the first reference value, whereinif it is determined that the change in the amount of aberration is lessthan the second reference value, said selecting unit selects the secondfunction.
 13. The apparatus according to claim 10, further comprising athird determination unit configured to determine whether the repeatcount of the number of times of the processing by said aberrationcorrection unit exceeds a third reference value, when the change in theamount of aberration is not less than the second reference value,wherein if it is determined that the repeat count of the processingexceeds the third reference value, said selecting unit selects thesecond function.
 14. The apparatus according to claim 10, wherein afterthe processing controlled by the control unit is repeated by apredetermined repeat count or a predetermined repetition time haselapsed from the start of the processing controlled by the control unit,said selecting unit selects the second function in accordance with themeasurement result obtained by said aberration measurement unit.
 15. Anon-transitory computer-readable storage medium storing a program forcausing a computer to execute a fundus imaging method defined inclaim
 1. 16. The method according to claim 1, wherein N=1, M=4, and L=6.